DENATURATION OF LYSOZYME BY N-ALKYLTRIMETHYLAMMONIUM BROMIDES IN ALKALINE-SOLUTION

The interactions of a homologous series of n-alkyltrimethylammonium br omides (C-8-C-14 TABs) with hen egg lysozyme have been investigated us ing microcalorimetry. The C-8 TAB does not interact with lysozyme wher eas the C-10-C-14 TABs interact endothermically and deactivate the enz yme. The endothermicity of the TAB-lysozyme interaction is in marked c ontrast to the exothermic interactions between n-alkyl sulfates and ly sozyme which have been attributed to specific binding between the anio nic sulfate head groups and cationic amino acid residues. The enthalpi es of interaction between the cationic surfactants and lysozyme follow sigmoidal curves characteristic of an interaction dominated by the en dothermic unfolding of the native structure. The enthalpy data have be en used to obtain the Gibbs energy and entropy for surfactant-induced denaturation. At pH 10 the Gibbs energy, enthalpy and entropy of denat uration are 17.9 +/- 4.2 kJ mol(-1), 148 +/- 15 kJ mol(-1) and 436 J m ol(-1) K-1 at 25 degrees C.